Hitherto, the silicon wafer which is used as the base board of an I.C. device has been made from a mono-crystal silicon block by the following processes. The silicon block is sliced by diamond cutting wheel to form a thin disc, the disc is then ground to form parallel surfaces, and is thereafter lapped on the surfaces thereof by isolated abrasive particles so that the disc has a thickness of 0.1 to 0.6 mm with mirror-like flat surfaces. The lapping step removes small cracks and the metamorphosed layer which is formed on the surface during the diamond wheel cutting and subsequent grinding steps. Such finishing process is necessary for the base plate to have a properly finished surface.
The finishing process requires a long processing time because the lapping operation can not be performed with too much pressure as it is necessary to avoid impregnation of abrasive particles in the surface of the chip. On the other hand, the metamorphosed layer must be removed. Thus, a long operation time is necessary for the lapping to remove the metamorphosed layer if lapping is done with low pressure. For example, it takes 35 to 40 minutes to reduce the thickness of the silicon wafer by about 25 .mu.m.
Accordingly, this invention provides a method and its apparatus which solves such inefficient silicon wafer lapping process.
According to the present invention, the wafer is subjected to a blasting step between the grinding and lapping steps, during which a slurry of silicon carbide and water is blasted against the surfaces of the wafer to reduce the roughness thereof prior to the lapping step.
The blasting apparatus of the invention, for carrying out the process, includes a blasting device for blasting slurry composed of silicon carbide and water, a silicon wafer fixture made of resilient material, the fixture being of a hollow cylindrical shape to support and confine the wafer, the hollow section having an inner diameter a bit larger than the diameter of the wafer and having a supporting mount in the middle of the hollow section for engaging the underside of the wafer, a drain passage being provided between the cylinder wall and the mount, the drain having an outlet through the cylinder wall, a board loading the fixture thereon, and a transfer device for feeding the board into a blasting chamber.